To adjust speed and tread easily, every tread-required means of transportation nowadays is equipped with a front gear, rear gear, chain and gear shift mechanism. However, the front gear, rear gear, chain and gear shift mechanism are structurally intricate and bulky, have a narrow gear-changing range, incur much transmission loss, and tend to jerk while shifting gear. Therefore, a stepless gear shift mechanism characterized by two grooved wheels operating in conjunction with a V-shaped belt is developed. However, the stepless gear shift mechanism has disadvantages, namely large volume of the grooved wheels and the V-shaped belt, and a narrow gear-changing range. Accordingly, the present invention aims to disclose a linear gear shift mechanism for chainless vehicles, such that the linear gear shift mechanism for chainless vehicles is structurally simple and compact, has a wide linear gear-changing range, incurs little transmission loss, and never jerks while shifting gear.